The present invention generally relates to instruments and methods for detecting and/or measuring concentration levels of substances. More particularly, this invention relates to a gas sensing method and instrument capable of detecting the presence of a gas, for example, a combustible gas, over a wide range of levels within an environment, and accurately measuring the concentration of the gas in the environment.
Gas detectors are widely used in various applications, nonlimiting examples of which include medical and emergency services and the mining and utility industries, to detect the presence of potentially harmful or dangerous gases, especially combustible hydrocarbon gases. Gas detectors typically use a thick-film metal oxide semiconductor sensor whose metal oxide film is reactive to the targeted gas and when reacted exhibits a change (usually a drop) in electrical resistance. The response of these sensors is nonlinear relative to the amount of targeted gas present, and as such typical gas leak detectors are very sensitive at low level concentrations, for example, up to about 10,000 ppm (1% by volume), and become less sensitivity at higher concentrations (generally at a few percentages of gas concentration) until the output eventually encounters signal saturation. As an alternative to semiconductor-type gas sensors, pellistors and other types of sensors, e.g., infrared (IR) sensors, having essentially linear responses may be used in gas detectors. However, their linear responses render these sensors not ideally suited for use as leak detectors requiring high sensitivity at very low gas concentrations (e.g., below a few percentages of gas concentration).
Gas detectors are typically equipped with a visual readout that provides a quantitative assessment of the gas concentration, typically in parts per million (PPM) and/or the percentage of lower explosion limit (% LEL) for the particular gas. Gas detectors can also be equipped with an audible device that generates a sound proportional to the sensed gas concentration. One example is an audible “tick” sound that increases in frequency or rate (ticks per second) proportional to the sensed concentration. As used herein, an “audible tick” refers to a variable repetition rate of audio pulses, each, for example, approximately 250 msec in duration, to which a human ear is very responsive. The tick rate alerts the user to the presence of a gas to which the sensor is sensitive and, prior to the onset of signal saturation, the relative amount of gas.
Because the responses of semiconductor-type sensors are nonlinear relative to the amount of targeted gas present, gas detectors are often equipped with an adjustment capability that enables the user to adjust the audible output to cover different ranges. FIG. 1 schematically represents one such technique that provides for manual adjustment of the audible output (resulting from increased sensed concentration) using a potentiometer. FIG. 1 shows a gas detection circuit 10 that utilizes a sensor 12, for example, of the nonlinear type described above. The analog output of the sensor 12 is interfaced with audio circuitry that contains a linear summing amplifier 14, a voltage-controlled oscillator (VCO) 16 and a pulse generator 18 that cooperate to convert voltage to a pulsed output, and an audio speaker 20 that generates an audible tick in response to the pulsed output. The analog output of the sensor 12 is amplified by the amplifier 14 before passing through the VCO 16, whose output is characterized by a frequency of oscillation varied by the applied analog (DC) voltage of the amplifier 14. The pulse generator 18 utilizes the oscillating analog output of the VCO 16 to generate the pulsed output (tick signal) based on the frequency of the output of the VCO 16. The pulsed output of the pulse generator 18 drives the audio speaker 20, which produces an audible “tick” whose rate or frequency is in proportion to the gas concentration sensed by the sensor 12. The sensor 12 interfaces with the audio circuitry by functioning as part of a resistive (voltage) divider circuit connected to the amplifier 14 in parallel with a potentiometer 22. A knob or wheel (not shown) can be conventionally used to make adjustments to the electrical resistance of the potentiometer 22. Both the sensor 12 and potentiometer 22 are connected to a suitable DC or AC voltage source (not shown). By making manual adjustments to the potentiometers 22, a user is able to adjust the frequency of the output of the VCO 16 to set an initial tick rate for the audio circuit, as well as make subsequent adjustments to the frequency of the output of the VCO 16 and resulting tick rate as may be desired, for example, as the tick rate approaches a saturation level at which the tick rate is no longer perceptible to the user.
FIG. 2 schematically represents an existing digital technique for providing manual adjustment of the output of a gas detector. A gas detection circuit 30 is shown in FIG. 2 as utilizing a sensor 32 interfaced with audio circuitry that contains a linear summing amplifier 34, an analog-to-digital (A/D) converter 35 that generates a digital output based on the amplified analog output of the amplifier 34, a microprocessor 36 that generates a digital output based on the digital output of the converter 35, a pulse generator 38 that converts the digital output of the microprocessor 36 to a pulsed output, and an audio speaker 40 that produces an audible “tick” whose rate or frequency is in proportion to the gas concentration sensed by the sensor 32. The capability for making manual adjustments to the tick rate is provided by a pushbutton 42 that enables a user to input commands to the microprocessor 36. While effective and economical, this technique does not quite provide an immediate and proportional adjustment to the audio output, which makes pinpointing the location and source of a gas leak rather cumbersome.
In view of the above, it can be appreciated that improvements would be desirable in the ability to adjust the tick rate of gas detectors, so that gas leaks can be quickly detected at low concentrations, and then adjustments to the tick rate can be made so that the location of the leak source (where gas concentrations may be much higher) can be more quickly identified.